The invention relates to a method for monitoring the state of the earthing contacts of a contactor controlled by an exciter coil, said contactor being operated as part of an isolation unit for galvanically isolating a voltage source from an electric consumer device connected to the voltage source.
Contactors controllable by means of an exciter coil are known in various embodiments in the prior art. For example, the German printed publication DE 10 2010 032 456 A1 discloses an electric contactor comprising main and auxiliary contacts as earthing contacts. Such contactors are particularly used in battery systems of at least partially electrically operated vehicles in order if necessary, for example in the case of a fault in the battery of the battery system or a vehicle accident, to galvanically isolate the battery from the vehicle or from a charging device for recharging the battery, i.e from an electric consumer device.
In so doing, there is the problem that the contactor ages with every switching process, i.e the earthing contacts become more highly resistive. Besides the mechanical stress, the reasons for this ageing lie with the isolation processes. If a contactor isolates high currents or respectively high currents flow across the earthing contacts of the contactor, a so-called burn-off results, in particular when an electric arc is formed between the earthing contacts. Because the burn-off leads to a reduction in the contact material of the earthing contacts, said earthing contacts wear with time, which leads to a limited service life of such contactors and makes them more highly resistive. In order to ensure the complete functionality of an isolation unit, the contactor or the contactors of the isolation unit must therefore be replaced at certain temporal intervals. It is thereby desirable to know at which point in time the contactors should be replaced. In order to do this, a monitoring of the contactors is required.
In order to monitor the functionality of a contactor, it is known to check or respectively monitor whether the earthing contacts are still open. A common problem here is that an earthing contact once welded or partially welded as a result of an arc formation can loosen again, for example due to vehicle vibrations, and the contactor has then a high contact transition resistance without the monitoring recognizing this fact. The high transition resistance can then lead to a significant increase in temperature of the earthing contacts when the electric currents are high. Currents of for example 100 A up to 400 A (A: amperes) thus flow over the earthing contacts during a normal driving cycle. In the case of a current of 200 A and a contact transition resistance of 1 mΩ (mΩ: milliohm), a power loss of 40 W (W: Watts) is already transferred via the earthing contacts. In the case of a current of 400 A, the power loss even amounts to 160 W. The earthing contacts can thus greatly heat up when there is a high electric contact transition resistance and high currents, whereby the risk of a case of thermal failure increases.
A further known monitoring option is to count the number of contactor activations and to assume wear to the contactor at a predetermined number of contactor activations that would require a replacement of the contactor. A significant disadvantage with this is that the actual condition of the contactor is not taken into account and thus the contactor is generally replaced too early.
In the case of battery systems used in at least partially electrically operated vehicles, it is furthermore known to monitor the so-called pack voltage, i.e. that voltage which drops across the battery cells of the battery system that are electrically connected to one another, and the so-called link voltage, i.e. the voltage applied on the vehicle side, in order to thus be able to make assertions about the contactors connected therebetween.
In the light of the above, it is the aim of the present invention to improve a method for monitoring the state of the earthing contacts of a contactor controllable by means of an exciter coil in particular such that assertions about the quality of the state of the earthing contacts can be made in an improved manner and such that a determination can thus advantageously be made in an improved manner as to when the contactor can no longer be used or respectively should be replaced.